CHANGELOG: Add build-sbf's --skip-tools-install flag

#### Problem

#3762 added the ability to skip installing the tools somewhere, but it's
not documented in the changelog.

#### Summary of changes

Add a line describing the change.

CHANGELOG.md

@@ -18,6 +18,8 @@ Release channels have their own copy of this changelog:
   * CLI:
     * Add global `--skip-preflight` option for skipping preflight checks on all transactions sent through RPC. This flag, along with `--use-rpc`, can improve success rate with program deployments using the public RPC nodes.
   * Unhide `--accounts-db-access-storages-method` for agave-validator and agave-ledger-tool
+  * SDK:
+    * `cargo-build-sbf`: add `--skip-tools-install` flag to avoid downloading platform tools. Useful for immutable environments like Nix.
 
 ## [2.1.0]
 * Breaking: